Dolby AC-4 is an audio format for distributing rich media content efficiently. AC-4 provides a flexible framework to broadcasters and content producers to distribute and encode content in an efficient way. Content can be distributed over a number of substreams, for example, M&E (Music and effects) in one substream and dialog in a second substream. For some audio content, it may be advantageous to e.g. switch the language of the dialog from one language to another language, or to be able to add e.g. a commentary substream to the content or an additional substream comprising description for vision-impaired.
In order to ensure a proper leveling of the content presented to the consumer, the loudness of the content needs to be known with some degree of accuracy. Current loudness requirements have tolerances of 2 dB (ATSC A/85), 0.5 dB (EBU R128) while some specifications have tolerances as low as 0.1 dB. This means that the loudness of an output audio signal with a commentary track and with dialog in a first language should be substantially the same as the loudness of an output audio signal without the commentary track and with dialog in a second language.
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